Get into geeky gifts that glow

Uranium marbles glow with a greenish hue under ultraviolet light, but they're said to be safe — despite the radioactive sticker on the container.

Glow-in-the-dark uranium marbles have emerged as the top Science Geek Gift of 2011, but you don't have to go radioactive to get that greenish glow.

To be sure, there's something slightly subversive about marbles that are slightly radioactive. "Definitely geeky, but non-geeks would also love them because they glow and have a risk factor appeal," one commenter wrote.

The totally unscientific tally was close: The margin of victory was less than 50 votes out of more than 3,000 cast. But the green glow of victory means that Richard is eligible to receive a pile of geek-friendly books, including "The Cult of Lego,""Science Ink,""The Physics Book" and "The Case for Pluto." Because Joel came so close, I'm sending him an autographed copy of "The Case for Pluto" as well.

Now, about that uranium: In the old days, pigments containing uranium used to be found in things ranging from ceramic tiles to dinnerware and glassware. Today, uranium isn't used as a coloring agent, but probably not for the reason you'd suppose. Natural, unprocessed uranium isn't all that radioactive — but because it's a heavy metal, it's as toxic as lead. And we all know what happened to lead paint. On the Health Physics Society website, Washington State University's Ron Kathren says "chemical toxicity is the overriding consideration" when it comes to limiting the use of natural uranium.

A healthier glowThe health risks of radioactive inks and paints have been known since the 1920s, due to the illnesses suffered by the "Radium Girls" who painted the dials on glow-in-the-dark watches. Today, few manufactured items make use of radioluminescence, which involves converting radioactive emissions into visible light. (Exceptions include some types of watch dials, keychains and gunsights that glow due to paints containing tritium or promethium rather than radium.) Virtually all of the glow-in-the-dark items you see today take advantage of electroluminescence, chemiluminescence or photoluminescence.

The key substances in most glow-in-the-dark items are phosphors, chemical compounds that are good at taking in energy and emitting it as light. Zinc sulfide and strontium aluminate are the most commonly used glow-in-the-dark ingredients, and new glow-in-the-dark compounds continue to be developed. They're relatively safe: That's why you see so many kids' toys that glow in the dark, as well as these geekier items: